NO317813B1 - Cementation mixture suitable for pressure cementing and pressure cementing method - Google Patents

Description

The invention relates to a cementing mixture suitable for pressure cementing, comprising a liquid phase and a solid phase which is constituted by a combination of solid particulate substances comprising cement, and at least one particulate additive which is finer than cement, the ratio of which is chosen to maximize the packing volume fraction.

Furthermore, the invention relates to a pressure cementing method.

Before initiating production from an oil well, a casing or spiral pipe is lowered into the borehole and cemented over all or part of its length. The cementing primarily prevents fluid exchange between the various formation layers around the borehole and prevents gas from rising through the annular space that surrounds the casing, or limits the inflow of water into the production well.

In EP 0621247, a cementing mixture is described, comprising a liquid phase and a solid phase which is made up of a combination of solid particulate substances comprising cement, and at least one particulate additive,

which is finer than cement, the ratio of which is chosen to maximize the packing volume fraction. Such cementing mixtures are useful in well cementing processes.

In some cases, it appears that the cemented annular space does not fulfill its primary function and fluids find a path through pores, micro-cracks or other voids in the cement or through a "micro-annulus" at the transition between the cement and the casing or between the cement and the formation. Holes or cracks may also have been produced on purpose, e.g. at the beginning of the well's life, but may become undesirable as exploitation of the well continues.

To eliminate such defects in the seal, a repair or pressure cementation must be carried out. During such work operations, a slurry of fine cement is injected under pressure into the cracks^ microcracks or other openings to be blocked.

The main technical difficulties associated with pressure cementing are also encountered during other operations, especially the placement of cement plugs e.g. to isolate a zone in the well for special treatment or for gravel pack cementation. This packing is used to filter the sand in unconsolidated formations while allowing oil to flow through.

After the exploitation of the well, it can. in the productive areas of the formations, brine is introduced and it becomes necessary to remove the gravel pack - with all the resulting difficulties - or to plug it by cementing through the gravel.

A cement slurry or mixture is a dispersion of solid particles, which are assumed to be spherical, in a fluid. The mixture can only penetrate into a crack if the largest particles are smaller than the crack. This is quite obvious, but one must take into account a feature well known to all specialists in the flow of suspensions, namely that a mixture can penetrate deep into a crack only if the diameter of the particles is three or more times smaller than the diameter of the crack opening .

A normal Portland cement contains particles with an average diameter of approx. 20 micrometers (um), the largest particles having a diameter of approx. 70 jim to 90 u.m. Under these conditions, the cementing mixture should be able to penetrate easily into cracks that have a width of e.g. 300 jim. This is certainly not the case in practice.

Fine or very fine cements have thus been proposed which have an average diameter of less than 10 µm or only a few micrometres, the largest particles not exceeding e.g. 30 jams. These cements, commonly called "microcements", have a relatively disappointing performance in terms of their ability to penetrate, even when thoroughly dispersed using common dispersing additives such as polyanions containing sulfonate groups.

As the reactivity of a cementing mixture further increases with the specific surface area of its particles, i.e. with its degree of fineness, it is not always wise to reduce the size of the cement particles too much, as there is a risk of the cement settling too quickly, before the placing operation is completed.

The purpose of the invention is to provide new compositions for pressure cements, especially for cementing operations at oil, gas, water, geothermal and similar wells, which compositions have improved penetration properties compared to compositions of a known type.

We have discovered that the poor results which have been obtained with compositions of the known type are usually due to an excessive increase in the viscosity of the cementing mixture due to fluid loss.

The fractures or other openings to be blocked by pressure cementing are all at least partially confined by porous walls (the formation around the borehole or hardened cement from a primary cementing) which presents a major difference compared to normal flow in a pipe. The slightly porous medium tends to dry out the cementing mixture by removing part of its water phase, which results in an increase in the viscosity of the mixture and thereby an increase in the friction with the walls and an obstacle to the further penetration of the mixture into the crack. This increase in friction tends to promote an exchange with the porous medium and thus produce a further fluid loss. The cementing mixture must be very stable because the introduction or injection into a narrow crack promotes sedimentation and the formation of free water which can lead to pseudochromatography, namely the particles of the mixture that have the greatest density are deposited at the entrance of the crack, which of course prevents a deeper penetration of the remaining mixture to be introduced into the crack. Furthermore, it is clear that the small amount of cementing mixture that succeeds in penetrating into the crack will not provide a sufficiently good quality cement, especially because its strength is very small.

It should be emphasized that the pressure or press cementing operations are always less favorable with regard to fluid loss because the exchange areas with formations or other porous media are much larger. For primary cementing (cementing the annulus around a casing), the area-tii-volume ratio is usually less than 1 and often about 0.4. In contrast, ratios of approx. 25 normal for pressure cementing operations.

The invention provides a cementing mixture suitable for pressure cementing, comprising a liquid phase and a solid phase consisting of a combination of solid particulate substances comprising cement, and at least one particulate additive which is finer than cement, the ratio of which is chosen to

maximize the packing volume fraction, characterized by that

- the cement has an average particle size of between 0.3 µm and 10 µm, - the particle additive is insoluble in the liquid phase, has an average particle size of between 0.05 µm and 0.5 µm and is selected from styrene-butadiene-rubber latexes, silica ash, manganese pigment ash, carbon black and polymer microgels, - the mixture comprises a dispersant and a liquid fluid loss additive, and - the nature and concentration of the liquid fluid loss additive and the combination of solid particulate matter are such that the API liquid loss for the mixture is less than 30 ml and variations in viscosity of the mixture due to fluid loss is minimized.

The fluid loss is preferably less than 20 ml and preferably less than 15 ml, the values being measured using the API (American Petroleum Institute) standard, Spee. 10, Appendix F.

It should be noted that the prior art has largely underestimated the importance of fluid loss for pressure cementing mixtures. Current recommendations based mainly on the work of Hook, F.E. and Ernst, E.A.

(SPE 15104, 1969, "The Effects of Low-Water-Loss Additives, Squeeze Pressure, and Formation Permeability on the Dehydration Rate of a Squeeze Cementing Slurry"), is that it should not go beyond 200 ml of API, e.g. in formations with extremely low permeability, 100 ml to 200 ml API in formations that are slightly permeable, and 35 ml to 100 ml API in formations that are very permeable (permeability greater than 100 milliDarcy). Most of the additives that cause a small fluid loss further increase the plastic viscosity of the cement mixture, while the main criterion in the known technique is the need for a plastic viscosity that is as small as possible to facilitate the penetration of the mixture into the cracks.

To the extent that fluid loss can be largely reduced, but not completely avoided,

it is advantageous that the rheology of the cementing mixture is as small as possible. The term "rheology" covers not only plastic viscosity, the meaning of which is understood by the above-mentioned prior art, but also to some extent mainly the yield strength of the mixture. Relatively large fluid losses are better tolerated when the initial rheology of the formula or mixture is small.

High rheology increases the pressure drop and thus the pressure that must be exerted against the fluid to force it into the crack. However, an increase in this pressure contributes to increasing fluid loss, which is very harmful as mentioned above. High yield strength also causes clogging in the crack to be filled.

Preferred compositions of the invention have a yield strength of less than 10 lbf/100 ft<2> (4.8 Pa), preferably less than 5 lbf/100 ft<2>

(2.4 Pa) and more advantageously less than 2 lbf/100 ft<2> (0.9 Pa). The plastic viscosity is preferably less than 100 cP (0.1 Pa s).

For this purpose, the compositions or mixtures according to the invention comprise a dispersant which reduces the rheology of the cementing mixture. Common dispersants can be used. Examples are polyelectrolytes, e.g. charged water-soluble polymers, especially polyanions containing sulfonate groups attached to a backbone consisting of one highly branched polymer, such as polymelamine sulfonate (PMS), polynaphthalene sulfonate (PNS), the condensation product of melamine/formaldehyde containing sulfonate groups, polystyrene sulfonate or lingosulfonates. Low molecular weight polyacrylic acids form another large class of ionic dispersants which are widely used in the construction industry, but which are little used in oil industry cements due to their great retarding effect. Other non-ionic dispersants such as cellulose derivatives, polyvinyl alcohol, low molecular weight polysaccharides etc. are also known.

Any work operation in an oil well requires a formula or a mixture that is stable over a relatively long time, and it is obvious that it would be pointless to produce a complicated formula or mixture where different additives are used, if the mixture that is introduced in the cracks was completely different. In particular, this requires that the cementing mixture has no or practically no tendency to segregate, which means the presence of very small amounts of free water. As indicated above, instabilities cause pseudochromatography in the crack to be blocked. Mixtures or compositions with an amount of free water of approx. 0 ml, e.g. less than 2 ml, measured under the conditions of API Spee 10 (Section 6 or Appendix M). Precipitation is preferably less than 5%, more advantageously less than 2%.

Since the density of the mixture during compression does not play a major role except for the strength of the hardened cement, the density of the pressure cementing mixture according to the invention can be adjusted quite freely as a function of the characteristics of the surrounding rock to balance the pressures exerted by the formation and the injected mixture. A mixture that is too dense risks destroying the rock like a fracturing fluid. If the density is too small, on the other hand, there is a risk of inflow of fluids that are outside. The density is adjusted in a known manner by setting the amount of water and by supplying a suitable amount of particulate aggregates such as silica, sand, industrial waste, clay, barite, etc.

The cement is preferably a micro cement, but it should be emphasized that in many cases a normal cement can be used as shown below. The term "microcement" means a cement formed from particles having an average grain size of between 0.3 µm and 10 µm. Microcements containing particles no larger than 30 µm are more preferred, with a particle size distribution such that at least 90% have a diameter of less than 20 µm and 50% have a diameter of less than 7 µm.

The chemical composition of microcements according to the invention can correspond to the composition of a Portland-type cement, a slag clinker, a Portland cement/slag clinker mixture or

Portland cement/silica ash liners.

In order to minimize both rheology and fluid loss, an addition of particles which are finer than the microcement and which are not soluble in the liquid phase is particularly preferred. The ratios of solids both to each other and as a function of their respective grain sizes are chosen so that the packing volume fraction (PVF) is maximized or at least that the ratios are such that the maximum PVF is nearly achieved.

The choice of the ratio of the solids in relation to the liquid (or mixing fluid) is such that the resulting mixture is in a state in which sedimentation is prevented, where the solid particles behave collectively as a solid, porous material, with the percentages of the different fractions are the same from the top to the bottom of the column, while particles of different sizes or different densities settle separately at different speeds outside this area. In practice, this limit value, which depends on the selected grain sizes, corresponds to concentrations of solid materials that are much greater than those used for cementing mixtures.

The volume ratio between this particulate additive and the cement is usually in the range of between 10% and 50%, preferably in the range of between 20% and 40%, and more advantageously close to 30%. The nature of the admixture is not critical provided it is compatible with the cement, is on average substantially spherical, without anisotropy, and has a grain size different from the grain size of the cement.

Under these conditions, it appears for an identical fluid loss, that the composition according to the invention has a smaller increase in viscosity than a normal composition without an additive. This is the case to a greater extent the greater the cement's density. A cement without an additive has, after a fluid loss of 100 ml, a viscosity equal to the viscosity of the fluid according to the invention when it has lost 130 ml, these values being given for mixtures with a density of 18 ppg (2.15 g/cm<J >).

A particularly preferred example of a particulate additive according to the invention consists of latexes, especially styrene-butadiene latexes, SBR (styrene-butadiene rubber) which is advantageously completely compatible with cements and can be supplied with grain sizes of between approx. 0.15 fim and 0.4 jam. These latexes are advantageously used together with a dispersant, especially if the cementing mixture has a solids content of more than 35% or 40%. Examples of dispersants are sulphonates such as polynaphthalene sulphonate (PNS) or melamine sulphonate, other anionic dispersants are also suitable.

Other particle additives which are characterized by grain sizes close to the grain sizes of SBR latexes can also be used with very good results. Special examples are particles made up of silica condensate of the silica ash type, a condensate of manganese oxides in pigment ash, some fine soot, carbon black, and some polymer microgels as a substance that controls fluid loss. In what follows, these particles are generally referred to as "very fine".

With the exception of latex and polymer microgels, these particles are not known as fluid loss control agents. Under the preferred conditions according to the invention, however, they can cause very small fluid losses even in the absence of any additive with the particular purpose of obtaining a small water loss.

The preferred compositions according to the invention can thus be seen to include:

• a fluid body, especially a water body,

• a dispersing agent in solution in the water phase and, if desired, other liquid additives known from the past, in particular antifoam agents and retarding agents; or more specifically cement hardeners,

• solid particles in the form of combinations from the following categories:

"fine" cement particles with an average grain size of between 0.3 µm and 10 µm (microcement), and

"very fine", particles with an average grain size of between 0.05 µm and 0.5 µm, e.g. a latex, a silica condensate of the silica ash type, a condensate of magnan oxides in pigment ash, some fine soot, carbon black, or certain polymer microgels such as substances that control a fluid loss.

In a particularly preferred variant of the invention, at least one third type of solid particles is obtained, these being 5 to 100 times smaller and preferably approx. 10 times smaller than the "very fine" particles. For these "ultrafine" particles, dispersed colloidal silicas or aluminas (average grain size 3 nanometers (nm) to 60 nm, preferably 15 nm to 40 nm) or nanolatexes can be used. This composition preferably comprises between 10% and 40% of "very fine" particles and between 5% and 30% of "ultrafine" particles, the percentages relating to the total volume of the solid particles in the composition. Compositions comprising between 50% and 75% microcement, between 15% and 40% "very fine" particles, and between 5% and 20% "ultrafine" particles are particularly preferred.

Preferably, in the selection of particles constituting the "fine", "very fine", and "ultrafine" particles, detached or separated particles are selected, i.e. they do not overlap to any extent, a criterion which can generally be considered to have been satisfied if the grain size curves, expressed in volume, are moved at least a distance of half a top part, where the particle size is shown logarithmically.

Preferred compositions or mixtures according to the invention as stated have rheological properties which are particularly remarkable, and which, even with very small amounts of fluid mixture, are very easy to mix and to pour out. For the same amount of solids, mixtures are obtained that are more liquid,

which is particularly advantageous in connection with pressure cementing because this facilitates the penetration of the mixture into the openings. Furthermore, the fluid losses are small, and in any case less critical than what is indicated above. It is thus possible to produce high density mixtures, especially of more than 1.8 g/cm<3> (15 ppg) which are perfectly suited for secondary cementing operations, and which combine a low tendency to fluid loss with a greater tolerance for this .

This point is particularly advantageous, because as indicated above, we have found that mixtures with high density according to the invention are less sensitive to fluid loss and thus very suitable for secondary

cementing work operations. Mixtures with high density are usually associated with higher viscosities and as such are usually avoided as it is obvious that high viscosity is not advantageous for good penetration into cracks or other narrow openings. However, when operating under multimodal conditions, the most favorable conditions can be achieved. Preferably, the mixture or formula according to the invention is a tetramodal combination comprising at least four types of particles with different grain sizes, and more advantageously a pentamodal mixture or composition which can further reduce the amount of liquid required. More complicated combinations are also possible, but are usually more expensive. It should be emphasized that whatever number of "modes" is used, detached or discrete grain sizes must be used, and a formula with a continuous spread of grain sizes must be avoided. Without deviating from the idea of the invention, it is of course possible, if necessary, to use materials of different nature, but with the same grain size, which materials will then together constitute a single "mode".

It should also be noted that the proposed grain sizes are only given as an indication, as other materials may be used, provided they satisfy the above criteria. It should also be noted that the calculation of PVF is a known, common technique and it is sufficient to note here that the components of a particle mixture must be divided into grain size fractions, as satisfactory results are obtained by dividing into e.g. about. thirty or ten fractions.

Even in the absence or presence of only small amounts of a substance that controls fluid loss, the multimodal formulas or compositions of the invention are highly resistant to fluid loss to the formation.

The density of the cementing mixture according to the invention can be set very precisely without the viscosity of the mixture being increased too much and thus without a compromise being made between its mixing and injectability, even in the absence or presence of only very small amounts of dispersants.

Incidentally, the characteristic of the invention is also apparent from the distinguishing features specified in the claims.

The following examples show the invention and describe other advantageous details or features thereof, without the scope of the invention being limited in any way. These examples are shown in the drawing.

Fig. I is a schematic diagram of an apparatus for testing a mixture

ability to penetrate cracks.

Fig. 2 is a diagram showing three curves comparing the evolution of the viscosity of monomodal, bi- and trimodal mixtures. Fig. 3 is a diagram showing grain size curves for latex (3A),

carbon black (3B), and nanolatex (3C) and a colloidal silica (3D) which are used as particle additives in mixtures according to the invention.

Measuring device

The apparatus shown in fig. 1 was used to simulate the flow of a cementing mixture in a crack. This device consists of a transparent Plexiglas plate 1 which is placed on a filter paper plate 2 on a porous support plate. A channel is formed between the transparent plate 1 and the filter paper plate 2 by means of an adhesive tape 3 of known thickness (160 µm, unless otherwise stated). Clamps (not shown) hold the plates together. The transparent plate has a hole 4.

The cementing mixture was introduced at 25 °C via the hole 4 by means of a syringe 5 which was extended by means of flexible plastic tubes 6 and driven by a small motor (not shown) to displace the plunger of the syringe at a constant speed. After the injection, the filter paper was dried and the distance traveled by the cement in the channel delimited by the strip was measured. The length of the plate between the hole 4 and the open end of the channel was 230 mm. The filter paper functioned as an intermediate link between the porous medium (the porous support plate) and a medium that was not porous (the transparent plate) and thus simulated the medium into which a cementing mixture is usually injected (natural formations, cement, etc.).

The apparatus was used to test different cement formulas prepared with mixtures whose characteristics are indicated below.

Microcement

The microcement used for the samples was a mixture of Portland cement and ground granules of slag clinker, type Spinor A-16, sold by Ciments d'Origny, France. Its chemical composition (expressed as a percentage by weight of dry matter) was as follows:

Its physical characteristics were as follows:

Density: 2.93 gem"<3>

Average particle diameter: 4.6 µm

Maximum particle size: 16 um

Specific surface area per weight unit determined by air permeability test (Blaine fineness): 0.8000 m<2>/g

Results obtained with a Malvern Mastersizer granulometric laser particle size analyzer:

Specific surface area: 1.5127 m<2>/g

Cement

These ordinary cement tests are carried out with a class G Portland cement, Cemoil (a trademark of Compagnie des Ciments Belges). This cement had the following chemical composition (indicated as weight % of dry matter): In the following table, the abbreviations used for the first three columns are the usual abbreviations within cement chemistry, i.e. C3A=3CaO.Al203; C4AF=4CaO.Al2O3.Fe2O3; and C3S=3eaO.Si02-

The physical characteristics were as follows:

Density: 3.23 gem"<3>

Average particle diameter: 25.24 µm

Maximum particle size: 150 p.m

Specific surface area per weight unit determined by air permeability test (Blaine fineness): 304 m<2>/g

Results obtained with a Malvern Mastersizer granulometric laser particle size analyzer:

Specific surface area: 0.3245 m<2>/g

"Very nice" <p>articles

SBR latex

Styrene butadiene latex (weight ratio 50:50), stabilized by a sodium salt of the condensation product of p-naphthalenesulfonic acid with formaldehyde. The percentages given for the compositions are based on the total latex volume, including stabilizer.

The grain size curve is shown in fig. 3 Å.

Average grain size 175.8 nm. 10.62% of the particles had a grain size less than or equal to 650 nm; 9.9% have a grain size greater than or equal to 267.9 nm. The maximum measured grain size is 500 nm.

Sweet

The grain size curve is shown in fig. 3B.

Average grain size 3.32 nm. 10% of the particles had a grain size less than or equal to 0.77 µm; 10% had a grain size greater than or equal to 26.99 (im. The maximum measured size of the particles was 80 nm.

Although the average size of the particles was significantly larger than for latex, carbon black gave fairly good results, presumably due to its relatively high proportion of fine particles.

Micro eels

A fluid loss control agent was obtained by chemically cross-linking a polyvinyl alcohol in accordance with that described in French patent application FR-A-2 704 219. The starting polyvinyl alcohol had a hydrolysis ratio of 88% (mol) and a molecular weight of 160,000. The cross-linking agent was glutaraldehyde, the theoretical cross-linking ratio was 0.19%. The percentages given for the cementing mixtures were based on concentrated 2.6% water polymer solutions.

These products could be processed as particles with an average grain size of approximately 70 nm with a maximum particle size in the order of several hundred nanometers. Direct size measurements could not be performed and size was estimated from other characteristics.

"Ultrafine" particles

Nanolatex

Average grain size 26.8 nm. 8.9% of the particles had a grain size of no more than 18.5 nm; 10.3% had a grain size of no more than 44.9 nm.

Maximum measured size: 90 nm.

The grain size curve is shown in fig. 3C.

Colloidal silica

Average grain size 24.8 nm. 8.3% of the particles had a grain size of no more than 13 nm; 10.3% had a grain size of no more than 44.9 nm.

Maximum measured size: 60 nm.

The grain size curve is shown in fig. 3D.

Dis<p>ersion agents

Two types of commercially available dispersants were used:

• Dl=sulfonated melamine - formaldehyde copolymer in water solution, 20% by weight;

• D2=45% by weight solution of sodium salt of polynaphthalene sulfonate.

Other additives

The compositions or mixtures according to the invention can also include common liquid additives such as:

• Antifoaming agents, e.g. based on tributylphosphonate, polypropylene glycol, dibutyl phthalate or polyorganosiloxanes. For the samples • given here, the antifoam was an aqueous silicone emulsion, 35% concentration. • Curing retarders, e.g. lignosulphonates - optionally partially desulphonated - of hydroxyl carboxylic acids or salts of these acids, especially tartaric acid, D-gluconic acid or heptonic acid or onanthic acid (English: heptonic acid). The agent used for these samples was purified lignosulfonates. • Liquid fluid loss control agents such as derivatives of 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

The rheological measurements were carried out after a treatment of 20 minutes at the measuring temperature (27 °C), using a rotary viscometer with an outer, rotating cylinder, model Chan 35 in accordance with API standard, paragraph 10, Appendix H. The other measurements were also carried out at the same temperature. Fluid loss tests were performed using filter paper over a standard grid in accordance with the API standard for microcements.

The units and symbols used were:

Concentration based on volume (l/kg)

Ty: yield strength (Pa).

PV: plastic viscosity (mPa s).

FL: fluid loss in 30 min (ml).

F W: free water (ml).

Length: penetration depth in millimeters using the sample described in fig. 1.

Sedimentation: difference in density between the top and bottom of a 250 ml column, expressed as a percentage.

EXAMPLE 1

Three cementing mixtures with a density of 14 ppg (1.667 g cm'<3>) were prepared from a microcement under API standard conditions.

The fluid loss control agent was a microgel and the dispersant was type D1.

The values shown in the table clearly show the influence of fluid loss on the cementing mixture's ability to penetrate a crack, even if - as is the case here - the addition of a fluid loss control agent is accompanied by a relatively large increase in the cementing mixture's viscosity. Cementing mix No. 1 with very good rheology, penetrated only approx. 40 mm.

For an essentially identical fluid loss, the cementing mixture penetrated deeper when the rheology was less (see No. 3 and No. 4).

EXAMPLE 2

For example 2, a liquid AMPS fluid was used for low fluid loss.

For sample No. 5, the microcement particles were the only solid particles in the cementing mixture. For sample No. 6, colloidal silica particles were added at an optimum concentration with respect to the cement to achieve an approximation of the maximum PVF.

The dispersant was D2. The concentration of the antifoam agent was 0.009 l/kg.

The colloidal silica particles greatly reduced fluid loss and resulted in superior penetration, although rheology was degraded; in particular, the viscosity was increased to a large extent with particles of this type.

EXAMPLE 3

In this series of tests, cementing mixtures with a density of 14 ppg (1.667 g cm"<3>) were prepared using a microcement. The concentration of the antifoam agent was 0.009 l/kg.

The fluid loss control agent was a latex as described in European Patent No. EP-A 0 091 377. The bimodal mixture of microcement and latex had a maximum PVF for a latex concentration of 0.13 l/kg.

~1: 230 mm is the maximum crack length. A length of 230 mm means that the fluid flowed directly through the crack and out the other end. The value of 200 mm was obtained with a crack of 120 µm.

The first conclusion that can be drawn from this sample, especially when compared to samples No. 7 and No. 8, is that a cementing mixture penetrated further into a crack when its yield strength was lower, for an identical fluid loss and plastic viscosities.

It should also be noted that compositions close to the maximum PVF, with smaller amounts of fluid loss control agent, were far superior to the other two.

The well-dispersed particle additive used under the preferred conditions of the invention (No. 8 and No. 9) gave very good penetration.

EXAMPLE 4

The results reproduced here were obtained with optimal three-modal compositions. The density of the cementing mixture was again 14 ppg (1.667 g cm"<3>) and the cement was a micro cement.

~1: Concentration of antifoam agent 0.035 l/kg.

~2: The sample was repeated with a gap of 120 u.m. Again, the cementing mixture flowed to the end of the slab opposite the injection end. With a slit of 60 um penetrated

the cementing mixture to a depth of 4 mm.

~3: This additive was solid and the concentration is given here in weight % of cement.

With these formulas or mixtures, all four of which were trimodal, both a low fluid loss and an excellent rheology, especially with regard to the dispersion, were achieved. In all cases, the cementing mixture penetrated into the gap without difficulty:

EXAMPLE 5

Three 14 ppg (1.667 g cm"<3>) cementing mixtures obtained from a regular microcement (M) or a regular cement were compared. These tests used a gap of 320 [im.

Sample 14M corresponded to a highly dispersed microcement recommended in the art for pressure work operations, but without a low fluid loss additive. Although the gap was 20 times larger than the largest particles in the microcement and the ratio between the thickness of the gap and the average diameter of the particles of the microcement was approx. 90, the 14M compound penetrated to a depth of only 150 mm. The cementing mixture 15C which was prepared from an ordinary cement of particularly low rheology, well dispersed and of low viscosity, only formed fingers, i.e. the cementing mixture only arrived at the entrance of the slot, but only penetrated to a depth of less than 1 mm. In other words, this was completely unsuitable for use in pressure cementing.

Sample 16C, again with ordinary cement with the same dispersion level, but using a trimodal system, where the various additives were added to give a PVF close to the maximum PVF, on the other hand gave a very satisfactory penetration, far better than that achieved by test

14M, which is a particularly remarkable result which perfectly demonstrates the advantages of the invention.

Example 4 shows particularly good results that are obtained with trimodal compositions. As shown in fig. 2, this remarkable behavior is primarily due to the relative insensitivity to fluid loss. The viscosity increase for a cementing mixture without solid additives (monomodal composition) after a fluid loss of 100 ml was given the reference point 100. Curves A and B show the fluid loss for the same viscosity increase for bi- and resp. trimodal compositions. The very great superiority of the compositions which include particulate additives can be seen. The superiority is even greater when the density of the cementing mixtures or formulations is greater.

Claims (14)

1. Cementing mixture suitable for pressure cementing, comprising a liquid phase and a solid phase consisting of a combination of solid particulate matter comprising cement, and at least one particulate additive finer than cement, the ratio of which is selected to maximize the packing volume fraction, characterized in that - the cement has an average particle size of between 0.3 µm and 10 µm, - the particle additive is insoluble in the liquid phase, has an average particle size of between 0.05 µm and 0.5 µm and is selected from styrene-butadiene-rubber latexes , silica ash, manganese pigment ash, carbon black and polymer microgels, - the mixture comprises a dispersant and a liquid fluid loss additive, and - the nature and concentration of the liquid fluid loss additive and the combination of solid particulate matter is such that the API liquid loss for the mixture is less than 30 ml and variations in the viscosity of the mixture due to fluid loss is minimized. 2. Cementing mixture according to claim 1, characterized in that the liquid fluid loss additive is soluble in the liquid phase. 3. Cementing mixture according to claim 1, characterized in that the fluid loss additive is insoluble in the liquid phase 4. Cementing mixture according to one of the preceding claims, characterized in that the mixture's pour point is less than 4.8 Pa. 5. Cementing mixture according to claim 4, characterized in that the mixture's pour point is less than 2.4 Pa. 6. Cementing mixture according to one of the preceding claims, characterized in that the average particle size of the particle additive is 1/5 - 1/100 of the particle size of the cement. 7. Cementing mixture according to claim 6, characterized in that the average particle size of the particle additive is approx. 1/10 of the particle size of the cement. 8. Cementing mixture according to one of the preceding claims, characterized in that the volume of the particle additive is between 10% and 50% of the volume of the cement. 9. Cementing mixture according to claim 8, characterized in that the volume of the particulate additive is between 20% and 40% of the volume of the cement. 10. Cementing mixture according to claim 9, characterized in that the volume of the particulate additive is approx. 30% of the volume of the cement.. 11. Cementing mixture according to one of the preceding claims, characterized in that it further comprises very fine (ultrafine) particles with a particle size of between 1/5 and 1/100 of the average particle size of the additive, and that the mixture comprises additive of 10% - 40% of the total volume of the solid particulate material, and very fine particles of 5% - 30% of the total volume of the solid particulate material. 12. Cementing mixture according to claim 11, characterized in that the very fine (ultrafine) particles have an average diameter of between 3 nm and 60 nm. 13. Cementing mixture according to claim 12, characterized in that the very fine (ultrafine) particles have an average diameter of between 15 nm and 40 nm. 14. Cementing mixture according to claim 12 or 13, characterized in that the solid particulate matter comprises 50% - 75% microcement, 15% ^ 40% additive and 5% - 20% very fine (ultrafine) particles, calculated in percentage by volume. "15. Pressure cementing method, characterized in that it comprises injecting a cementing mixture as stated in one of claims 1-14 under pressure into cracks, micro-cracks or other openings in a geothermal well or an oil, gas or water well.